Although dominance hierarchies were first studied in domestic chickens — hence the term “pecking order” — many different kinds of animals, fish included, fight among themselves to see who is boss. In the wild, these dominance hierarchies are usually formed between conspecifics, but in aquariums a hierarchy can often develop between the inhabitants, regardless of their species.

Why do fish establish dominance hierarchies?

Hierarchies develop when individual fish attempt to stake a claim for a share of the resources, which in most cases means food. The largest and most aggressive fish are capable of out-competing others so they take a disproportionate chunk for themselves, leaving the others to squabble for what is left. Royal grammas (Gramma loreto) live in groups on the undersides of reef ledges and feed on drifting plankton. The best spots for gathering plankton are always held by dominant individuals in the group, who aggressively defend their place against subordinate fish.

Although fighting for an elevated place in the hierarchy can be costly in the short run, the dominant fish gain their rewards over time, growing faster than lower-ranked fish and maturing sooner into breeding adults. However, subordinate fish also benefit from being in the group. Damselfish at the bottom of the pecking order grow comparatively slowly, but are less likely to be captured by predators because of the safety of being in a group.

How do fish decide on their place in the hierarchy?

Fish decide an order of precedence through aggression and direct competition from the dominant individuals at the top of the hierarchy down to the weaker subordinate fish at the bottom. Dominance is usually determined by size, age, and experience: larger fish boss small ones, older fish dominate younger ones, and fish that have been living in a particular locale for a period outrank newcomers.

But for otherwise equal fish, the hierarchy may be decided by a great deal of displaying and even out-and-out fighting, although this is by no means the only method they use. Simply by watching others fight, fish can decide whether they have any real chance against the combatants. If they decide that one or other of the fighters is a real bruiser, they can save themselves from injury by signaling submissiveness to these fish in future. Perhaps the best predictor of a fish's position in a dominance hierarchy is its size, but even among closely matched fish the order is usually established extremely quickly, often within 24 hours.

What about hierarchies based on their member's sexes?

A number of coral reef fish live in breeding groups where the sexes assume different positions in the dominance hierarchy. For example, some cleaner wrasses form so-called linear hierarchies, where each individual occupies a step on the ladder, dominant to those below it but subordinate to any above. (The alternative is the “despotic hierarchy,” sometimes seen in eels, where a single dominant individual aggressively rules the roost over all others, who are each of approximately equal rank). In cleaner wrasses, the hierarchy is topped by a single dominant male and the other fish in the pecking order are all females, whereas in clownfish, the reverse is true; the female is at the top of the pile and the males form the remainder of the hierarchy.